This invention relates to the field of electronic reflectometry and, more specifically, to the measurement of extremely fast time or frequency based electronic signals utilized in the testing of circuits or devices at millimeter wavelength frequencies. This invention is the result of a contract with the Department of Energy (Contract No. W-7405-ENG-36).
It has long been common practice in the field of electronics, when seeking to characterize an unknown circuit or device, to introduce a known signal to the circuit or device and to thereafter measure the response to that signal. Often, the necessary information can be obtained through analysis of the time-varying, periodic signal reflected back toward the source of the known signal by the circuit or device under test (DUT). This reflected signal is made up of discrete frequency components which can be analyzed for information on the circuit or device.
For devices operating at millimeter wavelength frequencies, conventional testing has involved hollow waveguide measurement systems. However, these systems provide only marginal measurement accuracy because of parasitic effects associated with the waveguide to chip transition, poor circuit stability when testing circuits or devices which are active below the waveguide's cut-off frequency, and the inherent bandwidth limitations of the waveguide itself. The maximum achievable frequency band coverage with such a system is currently limited to approximately 60 GHz.
Other prior art measurement systems, involving direct connection of a signal generator to a DUT with fast sampling of the reflected and incident signals, are severely hampered by time resolution and frequency bandwidth limitations. In the case of time domain reflectometry, commercially available equipment generally is unable to achieve a time resolution better than about 25 ps. For frequency-domain measurements, commercial equipment can attain bandwidths (error corrected) of only about 26.5 GHz in a coaxial system.
Recently, significant advances have been reported concerning photoconductive circuit elements (PCE). These on-chip photoconductive semiconductor devices are characterized as exhibiting low conductance in dark conditions, and high conductance (low resistance) in the presence of light.
Photoconductive circuit elements are fabricated from semiconductive material. However, for most applications, the materials are either gallium arsenide, silicon, or indium phosphide. For microwave applications, as in the present invention, the preferred material is gallium arsenide (GaAs).
In their natural states, when subjected to incident light energy, these semiconductors undergo a change to the conductance state through the instantaneous generation of electron-hole pairs. Upon the removal of the light energy, however, these electron-hole pairs require a relatively long period to recombine and return the semiconductor material to the low conductance state. Thus, the conductance response of the material to an extremely fast pulse of light from a laser would be a pulse with an extremely fast rise time, followed by a relatively long decay time constant on the order of 100 ps.
It has been found, though, that this decay time constant can be improved through damaging the semiconductor material. This damage is effected by the addition of foreign atoms into the semiconductor, or by subjecting the semiconductor to radiation, usually in the form of alpha particles, protons, or neutrons.
These techniques reduce the decay time constant of the semiconductor by creating additional electron-hole pair recombination centers in the material. Decay time constants on the order of 1-2 ps have been attained with radiation damaged gallium arsenide PCEs when mounted directly onto a semiconductor chip as a gap in a high speed transmission line. Such an application was reported by D. H. Auston, "Impulse Response Of Photoconductors In Transmission Lines," IEEE J. Quantum Electron., QE-19, 639-647 (April 1983).
The advent of lasers capable of subpicosecond or femtosecond optical pulse widths has led to the utilization of the fast response time PCEs as pulse generators and sampling gates in response measurements. Such an application was described by W. R. Eisenstadt, "On-Chip Picosecond Time-Domain Measurements For VLSI And Interconnect Testing Using Photoconductors," IEEE Trans. Electron Devices ED-32, 364-369 (February 1985). This article deals with the use of fast switching PCEs mounted onto semiconductor substrates as gaps between sections of microstrip transmission line for use as pulse generators and sampling gates in device test procedures. In FIG. 2 of the article, several PCEs are shown located immediately downstream from the pulse generator only for the purpose of verifying their characteristics. A CPM dye laser was employed to provide the optical pulses for the PCEs.
For actual test measurement, the Eisenstadt article disclosed the arrangement illustrated in FIG. 3 of that article, where the DUT is situated between the generator PCE and a single sampler PCE. In that arrangement, the sampler PCE samples the charge from the output waveform of the DUT when stimulated into conductance by a time delayed derivative of the laser pulse used to stimulate the generator PCE, and provides output directly to a lock-in amplifier. A lock-in amplifier is basically a frequency sensitive voltmeter, well known in the art. The article does not disclose the use of PCEs for reflectance measurements.
The above-described articles of Auston and Eisenstadt are incorporated herein by reference.
The time delayed derivative laser pulses utilized by Eisenstadt are generated from a second synchronous beam from the laser which is reflected multiple times by an assembly of mirrors mounted on a translating stage. These multiple reflections lengthen the path of the pulses, thereby introducing a delay in the time the pulses take to reach the surface of the sampling PCE. Of course, these time delayed pulses could as well emanate from separate lasers.
If such a system is to be used in reflectometry applications to provide an accurate characterization of the DUT, the incident and responsive or reflected pulse signals must be sufficiently separated in time. In practical applications, this is not possible, partially because of size constraints dictated by high frequency, on-chip applications, where the close spacings severely limit the time separation between incident and reflected signals. However, it is also the result of residual reflections caused by impedance mismatches.
Ideally, the pulse generaor end of the system would be fully impedance matched to the transmission line connecting it to the DUT. In reality, unavoidable residual mismatches will always exist, which serve to re-reflect the signal reflected from the DUT. These re-reflections, and other higher order echoes, contaminate the incident pulse signal when they overlap the incident signal in time at the point of sampling.
A further problem with the prior art relates to the undesirable effects presented by parasitic interference inevitably associated with conventional on-chip implementations of PCE generator and sampling systems. Such parasitics can appreciably limit time resolution and overall bandwidth of the system, as well as introduce a frequency domain based filtering aspect into the otherwise time domain based sampling process. Parasitics have the effect of confusing the relationship between the low frequency output signal of the sampler and the actual signal on the transmission line. Parasitics can also be the cause of bothersome discontinuities on the transmission line, which can result in measurement errors.
Still further, the prior art fails to compensate for the fact that the PCE generated pulses do not resemble ideal Delta functions, but rather may be modeled as pulses with abrupt rise times and single time constant exponential decays, due to the relatively long electron-hole pair recombination period. Such a model represents a pulse whose amplitude spectrum begins to drop off at a rate of approximately 6 dB per octave for frequencies above a certain critical frequency determined by the decay time constant associated with the pulse. This causes the signal-to-noise ratio to begin to deteriorate. Similar problems are associated with the sampling process, which must also contend with a non-Delta function at the sampling gate window.
It is an object of the present invention to provide photoconductive reflectometer equipment that will yield accurate measurement of very fast electromagnetic signals, with excellent time resolution and bandwidth.
It is a further object of this invention to provide reflectometer equipment which minimizes the effect of undesirable reflected signals and parasitic interference.
It is still a further object to provide reflectometer equipment which includes means for shaping the generated incident pulse so as to provide a usable bandwidth significantly greater than conventional equipment.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.